Remember "vertigo," the killer drug that causes mild euphoria? It swept through the Glades a few episodes ago, and Oliver didn't care until it caused his sister to wreck her car. Well, it's back. And once again, it's tangentially causing deaths, in the sense that one user wanders into traffic after taking it. So everyone's conclusion is that Count Vertigo must be making it, because he's the only one in the world capable of making a drug. But he's still locked up in an insane asylum and seems to spend most of his time babbling incoherently. But then he escapes! And when someone else dies of an actual vertigo overdose, Oliver figures out that the Count wasn't really involved. So he goes and kills the doctor at the insane asylum, along with his henchman orderly. Drug problem solved!
The drug problem causes some interpersonl troubles between Tommy and Oliver. The police think the drugs might have been sold at Club Verdant, just because the first victim had just been in the club and Tommy has a history of selling drugs. This leads to the police investigating the club's finances and discovering that Tommy gave ten thousand dollars to a corrupt zoning comminssioner so the building wouldn't get fully inspected. The police think he was trying to hide a drug operation, but we know he was worried about the Arrowlair, which is located directly below the club. Which is dumb. Anyway, he makes them get a search warrant and hides the Arrowlair, so everything is fine. Except that Oliver seemed to think for a second that Tommy might have been selling drugs, so Tommy quits in a huff. He's furious that Oliver, the vigilante murderer, could judge him. So now he's going to go work for his father, the evil Malcolm Merlyn.
Also, Diggle takes the middle part of the episode off from the main plot because he wants to pursue Deadshot. Oliver gets mad when Diggle misses the big aquarium shootout (there was a big aquarium shootout I didn't mention. Sorry about that), but then he decides to help out with that. And Oliver claims to have reformed, which means he decides not to kill he entirely harmless Count Vertigo. Way to go, Oliver!
The previouslies reveal that we'll be revisiting Count Vertigo, who we last saw as he was being dragged away, babbling incoherently. Also there might be some Deadshot. I realize they need to reestablish characters, but when a show only mentions a character once every four episodes, it kind of gives away what's going to happen. Why not mix it up a bit? Have a previously with no relevance at all, just to remind us that Thea and Roy still exist.
Okay, on with the show. We start in Club Verdant, which has a lot of people raving it up. I feel like the existence of Club Verdant is going to wreck this show's delicate balance between "episodes with club music" and "episodes with a charity gala." Everyone at this club seems really interested in waving their hands in the air, but it's not clear whether they just don't care. We zoom in on one woman, who's looking at their hands with an odd intensity. And she's still waving her hands around as she wanders into traffic and gets hit. She doesn't get hit right away; cars zip past her on either side for a while first. Maybe somebody could have stopped and helped her?
Inside the club, Tommy and Oliver gloat about how much money they're making. Oliver's already a billionaire, so this probably isn't moving the needle all that much. And their Lost and Found basket has lingerie in it. Guys, you do not want to handle underwear worn by the sort of person who loses her underwear at a club. Detective Quentin Lance comes in to tell them about the woman who got ran over. Incidentally, I'm going to start calling him "Quentin" because "Detective Lance" takes too long to type, and there are a lot of people named "Lance" on the show.
Now, back to the plot: the dead woman had a wristband from Club Verdant and capsules of Vertigo on her, so Quentin is curious if the drugs were sold at the club. Oliver says they don't allow drugs and Quentin doesn't seem to buy it. When he leaves, Oliver asks Tommy for a list of the employees so he can have Felicity check them for a drug record. Either this is the sort of thing that should have happened as part of a usual HR screen, or Oliver is going to violate their rights. Could be both, I guess. Tommy reminds him that the two of them have records as well. Oliver goes to his secret Arrowlair door (which isn't disguised as well as I'd like) and Tommy asks whether the vigilante finished off Count Vertigo. Oliver says he did. He's regretting not murdering him, I bet. Every night, he goes to sleep filled with remorse about every murder he didn't commit when he had the chance.